fix: Incorrect rate data shown on Grafana dashboards

[mattdy]

A caveat in advance - I'm fairly new to Grafana dashboards, and this is my first PR against this excellent project, so please forgive me if I've made incorrect assumptions or got something wrong!

I noticed that the Request rate and Request rate per client graphs on my Blocky Grafana dashboard were constantly increasing:

This implies that the total number of queries should be growing exponentially, but graphing blocky_query_total confirmed this wasn't the case, I was seeing a linear increase as you would expect.

With some digging, I think that the queries being used within these two graphs are incorrect, they should use irate instead of rate, and should set an min step (or interval) to give sensible data across restarts or missing steps. With making these two changes, my resulting graph looks a lot more believable:

This PR updates the two graphs in question to use irate and interval accordingly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.09%. Comparing base (55ab95e) to head (29cef8a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1948      +/-   ##
==========================================
- Coverage   90.13%   90.09%   -0.05%     
==========================================
  Files          92       92              
  Lines        6582     6582              
==========================================
- Hits         5933     5930       -3     
- Misses        500      503       +3     
  Partials      149      149              

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[0xERR0R]

Sounds reasonable, thanks for investigation. 👍